Antenna



June 10, 1952 w. slcHAK 2,600,274

ANTENNA Filed oct. 1o, 1945 e RELATNE POWER ONE wAY (ab) ATTORNEY Patented June 10, 1952 UNITED STATES PAEN'E FFICE (Cl. Z50-33.65)

6 Claims.

This invention relates to antennas for high frequency communication systems and particularly to means for obtaining an unsymmetrical radiation pattern. More specifically, the invention is directed to a, reflector shaped or modiiied to produce a radiation pattern in which the energy distribution closely approximates the relationship csc20 in one plane, generally a vertical plane, where is the angle measured from the axis of the undistorted or symmetrical beam (i. e., with an undistorted or conventionally shaped type of reflector) towards the modified side of the reilector.

In certain radio object locating systems, for example, ground search apparatus or low-altitude airborne sets, it is desirable that the distribution of energy in a vertical plane be such vthat the variation of energy density versus the radiation angle approximates a cosecant-squared function. This distribution provides generally uniform echo strength for targets disposed at substantially equal elevations with respect'to the antenna, regardless of whether the target is close in or distant, and it eliminates, to a great degree, the need for tilting the antenna to obtain satisfactory coverage of the area under observation.

VI-Ieretofore severaldiierent types of antennas have embodied reflectors shaped in such manner as to produce a csc20 radiation pattern, such as those disclosed in the copending applications of Ernest A. Martinelli, Serial No. 608,293, entitled Antenna and filed August l, 1945, and Serial No. 608,294, entitled "Antenna and led August 1, 1945, Patent No. 2,501,070 granted March 21, i950, and in my copending application, Serial No. 604,029, entitled Antenna and filed July 9, 1945, now abandoned.

With csc20 antennas as heretofore designed, it is diiiicult to shape the reflectors to procure a radiation pattern that not only approximates a csc20 function, but also is free of discontinuities that would appear as bumps on the graph of an experimentally observed curve or on the pattern produced and plotted in either polar or rectangular coordinates. Bumps or discontinuities in the radiation pattern that are 2 db. from maximum to minimum cause objectionable light and dark rings on a cathode ray tube indicator, such as PPI (plan position indication) scope, because of non-uniform ground illumination. Apparently these discontinuities in the csc`20 distribution pattern result from extra radiation from the unmodied portion of the reflector.

The present invention contemplates a means for eliminating the discontinuities in the pattern and thus for smoothing out of the bumps in the plotted curve of the radiation by means of a shelf added to the reflector.

Accordingly, it is one of the objects of the present invention to provide an antenna with means for substantially eliminating discontinuities in the radiation pattern produced thereby.

It is another object of the invention to provide in an antenna having arefiector'shaped to produce a csc20 radiation pattern, means for more closely approximating the cscz@ pattern.

For a better understanding of the invention together with other and further objects thereof, reference is had to the following description, taken in connection with the accompanying drawing.

in the drawing:

Fig. 1 is a front elevation view of an antenna illustrating one embodiment of the invention;

Fig. 2 is a sectional view along the line 2-2 of Fig. l; and

Fig. 3 is a graph showing the ideal cscz and the curves produced by a conventional csc20 antenna with and without the arrangement according to the present invention, the graph being in rectangular coordinates.

Referring now to Figs. 1 and 2 there is shown a reflector i0 which may be of any desired shape to produce a csc20 radiation pattern. Thus reflector l0 may be shaped according to the manner disclosed in any of the above mentioned copending applications. For simplification of description, the invention will be described herein as directed to a reflector I0 shaped generally as a parabolic cylinder and having a non-parabolic curved portion Il adjacent the upper edge thereof, the combination of the parabolic and nonparabolic portions being adapted to produce a csc20 pattern in a downward direction as indicated by the broken line pattern in Fig. 2.

As thus far described, reflector I0 when properly illuminated from a suitable radiation source i2, which may be of the dipole, horn or pillbox types, is adapted to produce a pattern which'is shown in rectangular coordinates in Fig. 3 as curve B, the ideal csc20 being shown as curve A. It will be noted that curve B has prominent bumps due to discontinuities caused by extra radiation from the parabolic lower portion of reeotor l0, these bumps being designated B1, Bz and B3. As will be understood, it is desired to eliminate these bumps. This maybe achieved according to the present invention by providing a shelf along the lower edge or the unmodified parabolic side of reflector I0. Thus a shelf I3 is mounted in any suitable manner, such as by screws or welding along the lower edge portion of reflector I0.

Shelf I3 comprises a flat sheet or strip of electrically conductive material disposed along the edge of the parabolic fportion of reflector I0. Shelf I3 extends beyond the lower edge of reflector I with its plane substantially parallel with the normal focal axis of the parabolic portion of reflector Il). However, if desired, shelf I3 may be slightly tilted in either direction relative to the focal axis to the position giving best results. The dimensions of shelf I3 are not critical: for example, shelf I3 may extend entirely the length of the edge portion of reflector I0 or only part way along the edge. Preferably shelf I3 extends more than half way along the edge of reflector I0 and may extend forwardly away from the reflecting surface of reflector I0 for a distance of from one to three wavelengths of the energy at the desired frequency of operation. It will be understood that when reflector I0 is shaped generally as a parabolic cylinder, shelf I3 will be a straight or flat strip whereas, for a reflector shaped generally as a paraboloid of revolution or having a curved edge, shelf I3 will be curved to conform with the outer edge portion of the reflector.

The effect of shelf I3 upon the radiation pattern is that the bumps that appear on curve B of Fig. 3 are almost entirely eliminated. The curve of the radiation pattern produced With a reflector having a shelf I3 according to the present invention produces a pattern substantially as indicated by curve C of Fig. 3.

While there has been described what is at present considered the preferred embodiment of the invention, it will lbe obvious to those skilled in the art that various changes and modifications may be made therein without departing from the f invention.

What is claimed is:

1. In an antenna for radiating and receiving waves of electromagnetic energy having radiating means and reflector means having a parabolic portion and a non-parabolic portion shaped according to geometric optics to produce a radiation pattern in which the energy distribution varies substantially as the square of the cosecant of the angle measured from the axis of the parabolic portion, means for eliminating irregularities in said pattern, said means comprising a sheet of electrically conductive material mounted along an edge of said reflector and extending forwardly from said reflector in a plane substantlally parallel to the normal focal axis of the parabolic portion of said reflector.

2. In an antenna including means for radiating ultra-high frequency radio energy and a reflector adapted to be illuminated by said rediating means, said reflector having a major portion of its reflecting surface of generally parabolic shape to direct radio energy when properly illuminated by said radiating means along a main path in a substantially symmetrical radiation pattern, a portion of the reflecting surface near and along a portie-n of the length of the edge of said reflector being of non-parabolic shape to direct said energy along a divergent path whereby an unsymmetrical radiation pattern is produced by said reflector that varies generally as the square of the cosecant of an angle measured from the axis of said parabolic portion, and means comprising an electrically conductive strip of material connected to said parabolic portion and extending forwardly thereof to substantially eliminate extra radiation from said parabolic portion and thus substantially eliminate irregularities in the said unsymmetrical radiation pattern.

3. An antenna comprising a reflector and a radiator to illuminate said reflector with ultrahigh frequency radio energy, said reflector comprising a parabolic portion and a non-parabolic portion, said portions cooperating to produce a radiation pattern that varies generally as the square of the cosecant of an angle that is measured from the axis of said parabolic portion, and means including an electrically conductive strip of material mounted along the edge of said parabolic portion and extending forwardly substantially parallel to the axis thereof to eliminate lrregularities in said cosecant squared pattern.

4. The antenna according to claim 3, wherein said strip extends forwardly of said reflector a distance of from one to three wavelengths of the radio energy at the operating frequency.

5. In an antenna including radiating means and a reflector adapted to be illuminated by said radiating means, said reflector having a major portion of its reflecting surface of generally parabolic shape to direct radiant energy when properly illuminated by said radiating means along a main path in a substantially symmetrical radiation pattern, a portion of the reflecting surface near and along a portion of the length of the edge of said reflector being of non-parabolic shape to direct said energy along a divergent path whereby an unsymmetrcal radiation pattern is produced by said reflector that varies as the square of the cosecant of an angle measured from the axis of said parabolic portion, a longitudinal strip of electrically conductive material mounted along the edge of said parabolic portion and extending forwardly of said reflector for a distance of from 1 to 3 wavelengths of the radiant energy at the desired frequency of operation, said strip serving to substantially eliminate extra radiation from said parabolic portion and thus substantially eliminate irregularities in the said unsymmetrical radiation pattern.

6. In an antenna, reflecting means haw'ng such shape as to produce a radiation pattern of a configuration that varies generally as the square of the cosecant of an angle measured from the axis of said reflecting means, means for illuminating said reflecting means by ultra-high frequency radio energy, and means including an auxiliary reflector made of electrically conductive material andmounted on one edge of said reflecting means and extending forwardly thereof for eliminating irregularities in the radiation pattern produced by said reflecting means.

WILLIAM SICHAK.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,061,508 Dallenbach Nov. 17, 1936 2,170,028 Kohl Aug. 22, 1939 2,270,965 Peterson Jan. 27, 1942 2,281,196 Lindenblad Apr. 28, 1942 2,287,538 Peterson June 23, 1942 2,350,644 Alexander June 6, 1944 2,421,988 Brown June 10, 1947 

